The term “lens” relates in particular to an optionally-correcting lens suitable for mounting in the frame of a pair of eyeglasses. Such an ophthalmic lens can present conventional eyesight correction, antireflection, anti-dirtying, anti-scratching functions, for example.
U.S. Pat. No. 5,886,822 discloses an ophthalmic lens presenting a projection insert. Such a projection insert is constituted by an optical imager for shaping light beams coming from an electronic and optical system that generates light beams from an electronic signal of the miniature screen, laser diode, or light-emitting diode (LED) type. The optical imager directs the light beams towards the eye of the wearer to enable the information content thereof to be viewed.
FIG. 1 is a plan view of such a known ophthalmic display.
In the lens 300 there is embedded an imager 400 constituted by a prism 401A, a backing prism 401B, a quarterwave plate 404, and a Mangin mirror 403. The combiner includes polarization separation treatment 402 that can be implemented in the form of a deposit of thin layers.
An electronic signal conveying information is delivered to a miniature screen by a cable that is not shown. The miniature screen 320 is illuminated by a back-lighting projector, and responds to this signal by generating a pixallized image corresponding to the information.
A light beam coming from the miniature screen and following a path that is represented by a dashed line is transmitted via a lens 360 and a mirror 325 within the ophthalmic lens inside which it passes to reach the polarization separator treatment 402. The polarization of the light beam emitted by the screen is oriented in such a manner as to lie in the plane of incidence of light rays on the polarization separator treatment 402. It is said to be oriented in the P direction. The light beam then propagates through the backing prism 401B, then through the quarterwave plate 404, and then to the Mangin mirror 403 where it is reflected to pass back through the quarterwave plate in the opposite direction. The role of the Mangin mirror is to produce an enlarged image I of the screen and to position it in such a manner that is at a comfortable viewing distance for the user. Commonly, this viewing distance is adjusted so that the image appears to the user as though it were situated 1 meter (m) ahead. Furthermore, the apparent size of the image may be about 12° along a diagonal, depending on the characteristics of the imager.
The quarterwave plate 104 has its axes oriented at 45° to the polarization of the light beam. Thus, on the first passage of the light beam, it comes out in a circular polarization state. Finally, at the end of the second passage, the light beam is in a linear polarization state, but oriented at 90° to its initial polarization. In this way, when the light beam reflected by the Mangin mirror 403 has passed a second time through the quarterwave plate 404, it encounters the polarization separator treatment 402 where it then possesses a polarization direction that is perpendicular to the plane of incidence, commonly written S. It is thus reflected with high photometric efficiency to the eye of the wearer who thus sees the enlarged image I of the miniature screen 320 via the Mangin mirror 403.
Such a display presents the following problems.
The insert 400 substantially occupies a cube of area equal to the frontal area of the insert, in which area the view of the environment is disturbed. Outside the cube, the wearer of the eyeglasses can see the surroundings through the lens 300.
It is found that the information image I displayed by the system suffers from a loss of contrast due to the superposition of light coming from the outside environment. This phenomenon is particularly noticeable when the information eyeglasses are used outdoors.
Contrast is defined as follows:C=(Ion−Ioff)/(Ion+Ioff)where Ion is the intensity received by the eye when looking at an information image placed in front of the surroundings, and Ioff is the image received by the eye when looking at the surroundings without an information image.
Furthermore, the polarization separator cube contained in the lens is visible from the outside which produces an unattractive appearance effect. This is due to the polarization separator multilayer treatment that allows only 50% overall of the non-polarized ambient light to pass through.